Various systems use devices based on magnetic materials, such as computers, computing components, memory, and so forth. Magnetization of materials can result in use of less electrical power and storage of information even when the electrical power is removed. Magnetic sensors can sense magnetic fields and provide output signals representing the magnetic fields.
Recently, ferromagnetic thin-films and other layers have been developed which can yield a magnetoresistive response in the range of an order of magnitude or more as compared with the magnitude of the magnetic field due to anisotropic magnetoresistive response. This response is sometimes called a “spin valve effect” because electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in these layers are parallel as opposed to non-parallel orientations.
Magnetic field sensors have an extremely wide range of application. Such sensors can be used widely in scientific and research communities in areas as diverse as magnetic resonance imaging and fundamental physics. Such sensors can also employed, for example, in the mining industry for conducting wide area surveys, and even for ground tracking in airports. Magnetic field sensors have been developed with dimensions ranging from a few microns to tens of microns which can provide a response to the presence of very small external magnetic fields. Some such sensors have been developed which utilize the spin valve effect, or the changes in material properties as an effect of a magnetic field on an electron spin in the material. An electron spin is an intrinsic property of electrons. Electrons have intrinsic angular momentum. The angular momentum or spin of an electron can be an up or down spin. Previous magnetic field sensors have been able to measure magnetic fields using the electron spin properties, but such sensors have had limited accuracy, experience signal drift, operate at limited temperature ranges, and suffer from material degradation which also negatively affects sensor accuracy and performance. Further, such sensors have often been expensive and difficult to fabricate and provide effective magnetic field sensing only when oriented at a specific angle or small range of angles with respect to a direction of the magnetic field.
There is a wide range of research activity associated with magnetic field sensors. A number of proposals to use spin dependent electronic processes in inorganic semiconductors exist. See, for example, A Honig & M Moroz, Precision absolute measurements of strong and highly inhomogeneous magnetic fields, Review of Scientific Instruments 49,183 (1978) and A. Jander & P. Dhagat, Solidstate magnetometer using electrically detected magnetic resonance, Journal of Magnetism and Magnetic Materials 322, 1639-1641 (2010), both of which references are incorporated by reference herein in their entirety. However, these processes result in very small current changes at moderate magnetic fields, whilst not providing benefits of organic semiconductors, such as flexibility and cost effectiveness. Some efforts have also been made to use magnetoresistive effects in organic semiconductors as field sensors. However, such approaches are limited to small magnetic fields (i.e., <50 mT), and these approaches do not provide an absolute measurement of the field. In these approaches, the sensor output, usually the resistance of the sample, in a fixed magnetic field varies with both temperature and device current.